A circle with circumference ${20}$ has an arc with a $72^\circ$ central angle. What is the length of the arc?
Solution: The ratio between the arc's central angle ${\theta}$ and $360^\circ$ is equal to the ratio between the arc length ${s}$ and the circle's circumference ${c}$. $\dfrac{{\theta}}{360^\circ} = \dfrac{{s}}{{c}}$ $\dfrac{{72}^\circ}{360^\circ} = \dfrac{{s}}{{{20}}}$ $\dfrac{1}{5} = \dfrac{{s}}{{20}}$ $\dfrac{1}{5} \times {20} = {s}$ $4 = {s}$ ${20}$ ${72^\circ}$ ${4}$